There are many applications for fumed silicas of extremely fine particle size in which it is convenient to apply the fumed silica in the form of an aqueous colloidal dispersion. Such applications include paper coatings, the sol-gel process for the manufacture of optical fibers and quartz glassware, and thermal insulation. Aqueous colloidal dispersions of fumed silica are also utilized for frictionizing and polishing. There are also many occasions where it is convenient to densify fumed silica for storage or transport by combining the fumed silica with water to form an aqueous colloidal dispersion.
Fumed silica is generally produced by the vapor phase hydrolysis of chlorosilanes, such as silicon tetrachloride, in a hydrogen oxygen flame. The overall reaction is: EQU SiCl.sub.4 +2H.sub.2 +O.sub.2 .fwdarw.SiO.sub.2 +4HCl
In this process, submicron sized molten spheres of fumed silica are formed. These particles collide and fuse to form three dimensional branched, chain-like aggregates, of approximately 0.1 to 0.5 micron in length. Cooling takes place very quickly, limiting the particle growth and insuring the fumed silica is amorphous. These aggregates in turn form agglomerates ranging in size from 0.5 to 44 microns (325 US mesh). Fumed silicas generally have very high purity, with total impurities, in many cases below 100 ppm (parts per million). This high purity makes fumed silica aqueous dispersions particularly advantageous for many applications.
Another consideration for many applications is the removal of grit from the aqueous colloidal dispersion of fumed silica since grit is a major source of impurities. Grit can also interfere with many applications of the dispersion. For example, in coagulation of latex rubber, grit will lead to the formation of defects in the structure of the rubber, and in the polishing of semiconductor single crystals grit can cause scratching. Thus it is generally desirable that the aqueous dispersion be of high purity. One method for increasing purity is to pass the aqueous colloidal dispersion of fumed silica through a filter, also referred to as filtering, to remove grit and other impurities. In order for an aqueous colloidal dispersion of fumed silica to be filterable, the viscosity of the colloidal dispersion must be low enough, and the colloidal dispersion must be non-dilatant, to enable the colloidal dispersion to pass through the desired filter. For the purposes of the present invention, a non-dilatant dispersion is a dispersion which will pass through a filter having a pore size of 1000 microns or smaller.
As described above, the ability of a dispersion to pass through a filter is also related to the viscosity of a dispersion. The finer the filter, i.e. the smaller the size of the pores of the filter, the lower the viscosity of the aqueous colloidal dispersion of fumed silica must be to pass through the filter. As will be appreciated by those of ordinary skill in the art, to increase purity, the aqueous colloidal dispersion of fumed silica should be passed through as fine a filter as possible. Thus it is generally advantageous to produce aqueous colloidal dispersions of fumed silica with low viscosities. For the purposes of the present invention low viscosities are viscosities below about 1000 centipoise.
Additionally, in order to be useful for the applications listed above and other potential applications the aqueous colloidal dispersion of fumed silica cannot gel into a solid. The ability of the aqueous colloidal dispersion of fumed silica to resist gelling is generally referred to as the stability of the aqueous colloidal dispersion. More stable aqueous colloidal dispersions will not gel as soon as less stable aqueous colloidal dispersions.
Generally a stabilizer, such as an alkali or base, is added to an aqueous colloidal dispersion of fumed silica to increase the stability of the colloidal dispersion. Thus, most generally known stable aqueous colloidal dispersions of fumed silica are actually aqueous colloidal dispersions of fumed silica and stabilizer. These aqueous colloidal dispersions of fumed silica are known to contain amounts of fumed silica of 30%, 40% and even up to 70%, by weight. For example, U.S. Pat. No. 2,984,629, to Loftman et al., hereinafter "Loftman", discloses an aqueous colloidal dispersion of fumed silica and alkali having a fumed silica concentration up to about 40% by weight. G.B. Patent No. 1,326,574, to Diether, hereinafter "Diether", discloses an aqueous colloidal dispersion of fumed silica and stabilizer having a fumed silica concentration up to about 70% by weight. The stabilizer in Diether is also an alkali.
However, in generally known aqueous colloidal dispersions of fumed silica, not containing an alkali or stabilizer, fumed silica concentrations of greater than about 30% by weight, result in an unstable colloidal dispersion which quickly gels. Also, in these generally known aqueous colloidal dispersions, as the fumed silica concentration of the aqueous colloidal dispersion nears 30%, by weight, the viscosity, and dilatancy of the aqueous colloidal dispersion increase to the point that passing the aqueous colloidal dispersion through a filter to remove impurities becomes very difficult.
For certain applications, however, it is desirable to have an aqueous colloidal dispersion of fumed silica without the presence of alkali and/or stabilizer where the concentration of fumed silica is greater than about 35% by weight. However, in heretofore generally known processes for producing aqueous colloidal dispersions of fumed silica without a stabilizer, having a fumed silica concentration greater than about 30% by weight, the aqueous colloidal dispersions produced are unstable and quickly gel. For example, U.S. Pat. No. 4,042,361 to Bihuniak, et al., hereinafter "Bihuniak", discloses an unstable aqueous colloidal dispersion of fumed silica without an alkali or stabilizer, having a fumed silica concentration up to about 45%, by weight. The stability of this colloidal dispersion, as disclosed in Bihuniak, can be measured in minutes, making the dispersion untransportable and unpractical for most purposes. Even Bihuniak, for the purposes of his invention, teaches the use of an aqueous colloidal dispersion of fumed silica having up to only about 30% by weight.